The most common type of waveguide propagates signals in only one specific electromagnetic field pattern or mode, out of an infinite number of possible modes. Single-mode operation occurs because the waveguide is designed so that signals are in a frequency band which is sufficiently low that only the mode with the lowest "cutoff frequency" can exist and no other mode can propagate. If other modes were allowed to propagate, signal energy could couple into and out of various modes substantially distorting the signal. Such a "conventional waveguide" is compact and easy to design, model and use. Unfortunately, maintaining only the lowest-cutoff mode in a given frequency band requires restriction of the waveguide cross section dimensions, and this, in turn, restricts power carrying capacity and limits the lowest achievable signal attenuation. As a result, design of some systems requiring microwave or millimeter wave signal transmission with high power or very low loss may be difficult or impractical.
An alternative type of waveguide is generally called "overmoded" in which a higher order mode is used, i.e. a mode which does not have the lowest cutoff frequency. Because other (unwanted) modes are also capable of existing as well as the desired transmission mode, this type of waveguide must feature internal structures which suppress the unwanted modes. Because internal structure, rather than restriction of cross section dimensions, is the basis for suppressing all but the desired mode, overmoded waveguide cross section can, in principle, be made arbitrarily large for a corresponding increase in power capacity and decrease in signal attenuation. Unfortunately, this type of waveguide, with unwanted mode suppression, is difficult to model and design, and its cross-sectional dimensions may not be amenable to compactness without significant design optimization.
Historically, the more successful type of overmoded waveguide supports the circular TE.sub.01 mode and uses either a dielectric lining or dielectric sheathed helix of insulated wire inside the circular cross section waveguide for suppression and decoupling of unwanted modes, e.g. see A. E. Karbowiak "Trunk Waveguide Communication", Chapmen and Hall Ltd. 1965. Both versions of overmoded TE.sub.01 waveguide were originally developed and tested for millimeter band (60-100 GHz) trunk line tele- communications between cities. Application of overmoded waveguide technology for high power and/or low loss transmission in microwave or millimeter wave radio communications and radar has also been suggested and developed to a limited degree, e.g. see R. M. Collins "Practical Aspects of High Power Circular Waveguide Systems" NEREM Record, Session 24, pp 182-183, (1962). However, more extensive use has been limited apparently due to limitations in optimization design modeling within the limits of available materials and manufacturing methods.
The difficulty in modeling overmoded waveguide is primarily related to the effects of design characteristics on mode coupling phenomena. Such modeling is necessarily numerical. Further, trends towards design optimization are difficult to affix and can be substantially different for different design situations. For example, changing a design parameter value, e.g. dielectric constant, can suppress one mode while causing significant coupling of another mode or even render the waveguide inoperable. A significant amount of effort has been expended in developing the theory and in performing computations to develop a practical design for an overmoded waveguide, as revealed in the following references:
"Lined Waveguide" by H. G. Unger, Bell System Technical Journal, Vol. 41, No. 2, March 1962, pp. 745-768;
"Helix Waveguide Theory and Application" by H. G. Unger, Bell System Technical Journal, Vol. 37, No. 6., September 1958, pp. 1599-1663;
"Normal Modes and Mode Conversion in Helix Waveguide" by H. G. Unger, Bell System Technical Journal, Vol. 40, No. 1, January 1961, pp. 255-280;
"Helix Waveguide" by S. P. Morgan and J. A. Young, Bell System Technical Journal, Vol. 35, No. 6, November 1956, pp. 1347-1384;
"Winding Tolerances in Helix Waveguide" by H. G. Unger, Bell System Technical Journal, Vol. 40, No. 2, March 1961, pp. 627-643; and
"Normal Mode Bends for Circular Electric Waves" by H. G. Unger, Bell System Technical Journal, Vol. 36, No. 5, September 1957, pp. 1292-1307.
"Normal Modes in Overmoded Dielectric Lined Circular Waveguide" by J. W. Carlin and P. D'Agostino, Bell System Technical Journal, Vol. 52, No. 4, April 1973, pp. 453-486.
However, unfortunately, this prior work cannot be directly extrapolated to compact, optimized bends, structures with less overmoding, and limited dielectric material selection for high power application in radar and radio communications. Accordingly, the need exists for practical circular overmoded waveguide of optimum design.